We propose to study the genetic system and the replication cycle of murine leukemia virus (MuLV) by using temperature sensitive (ts) mutants. The following methods will be used to achieve our objectives: 1. Grouping of the ts mutants isolated by complementation tests; 2. Characterization of ts mutants from each complementation group using biological, biochemical, electron microscopic techniques; 3. To study the mechanism of recombination using ts mutants. Mutants isolated will also be used to study other biological properties and virus-cell interaction of MuLV.